1. Field of the Invention
The present invention relates generally to a wet chemical processing apparatus and more particularly to an apparatus wherein the item to be processed is moved through a bath of liquid.
2. Description of the Prior Art
Processing items of manufacture by immersing them in a liquid bath is a well established manufacturing technique. In many instances, the nature of the item being processed and the nature of the process being performed on it interact cooperatively so that the desired result is easily achieved. An example of such a process is the chemical stripping of paint from a metallic object. Presuming that a chemical were used which would dissolve only the paint and not the metal, the stripping process terminates when all the paint has been removed from the metal. The uniformity of the process and the quality obtained in the treated item are essentially controlled by the cooperative interaction between the processing chemical and the characteristics of the item to be processed. The quality of the result produced by this process is almost independent of the nature of the apparatus in which the processing is performed. However, other chemical processes exist which are more demanding of the apparatus in which the processing takes place. An example of a process which places severe demands on the processing apparatus is the development of exposed photographic film. In that process both the chemical compositions of the baths and the duration and nature of the exposure of the film to the liquids in those baths greatly influences the quality of the result produced by the process.
As with the photographic industry, the semiconductor industry is vitally dependent upon liquid bath chemical processing. The processes used within that industry span the entire spectrum from processes which are highly tolerant to the nature of the apparatus, e.g. rinsing operations, to others which are highly sensitive to the nature of the apparatus, e.g. chemical etching. Perhaps one of the most dramatic examples of such a demanding process is the etching of silicon wafers.
Silicon wafers are a disk-shaped item approximately 0.025 inches thick and range between one and five inches in diameter. They are initially fabricated by slicing from a cylindrically-shaped single crystal boule of extremely pure silicon. Wafers are sliced from the boule by sawing, which chips the edges of the wafer and introduces both damage and stress to the wafer's circular planar surfaces. This damage and stress can be reduced or eliminated by chemically etching the wafers after slicing.
Silicon wafers are etched by submersion in a mixture of nitric acid and hydrofluoric acid buffered by the addition of acetic acid and heated to a temperature between 140.degree. and 180.degree. F. Introducing silicon wafers into this caustic material causes a violent chemical reaction which removes up to five thousandths of an inch of material per minute from the surface of the wafer. However, in addition to removing material, this reaction also produces gas bubbles on the wafer's surface. The presence of these bubbles interferes with the liquid etchant's contact with the surface of the wafer. If the wafer remains stationary while submerged in the etchant, these bubbles cause nonuniform etching and the process produces an unacceptable surface on the wafer. It is, however, possible to obtain an acceptable result with this process by supporting the wafer vertically on its edge while immersed in the liquid bath and rotating the wafer thus immersed. Under these processing conditions, the effects of the bubbles are spread more uniformly over the entire surface of the wafer. This seemingly simple process of rotating the wafer while it is submerged in the liquid bath raises technical problems because of the hostile environment in which the processing apparatus must reliably operate.
Current devices used for chemical etching of silicon wafers employ an open carrier into which a number of disk-shaped silicon wafers are loaded in a parallel orientation. This assembly with the carrier loaded with wafers is then lowered into the acid bath and rotated thereby rotating the wafers. The means used thus far to impart the rotary motion to the carrier employ an electric motor located outside the bath. The rotation of that motor is then either directly coupled to the carrier by means such as a polyvinyl chloride chain or by means of a set of parallel rotating rods passing horizontally through the walls of the vat containing bath. These prior devices, while successful at rotating the wafers, have experienced reliability and maintenance problems with mechanisms used to impart rotary motion arising from the hostile environment in which they must function.